Hydrostatic transmission



Filed May IS, 1965 DR AIN CHAMBER INVENTOR. il l GEORGE F QUAYLE ATTURNEY United States Patent O 3,304,710 HYDRSTATIC TRANSMISSION George F. Quayle, Jenkintown, Pa., assigner to Eaton Yale & Towne Inc., a corporation of Ohio Filed May 3, 1965, Ser. No. 452,816 16 Claims. (Cl. 60--ll9) This invention relates to a hydrostatic drive, particularly adapted for an industrial truck. More particularly, this invention relates to the construction shown in my copending application Serial No. 290,552, led June 25, 1963, and entitled Hydrostatic Drive for Industrial Truck, now U.S. Patent No. 3,182,444, issued May l1, 1965.

The basic concept of the .construction of this earlier application embodies the utilization of a control means responsive to the speed of an engine to effect the drive of a hydraulic motor by a hydraulic pump driven by the engine whose speed is controlling. Thus, the very engine that drives the hydraulic pump controls the actuation of the hydraulic motor by the hydraulic pump, merely through the speed control of the engine. This makes it possible to place the hydraulic pump and the hydraulic motor in a circuit with the hydraulic motor actuated through the fluid moved by the pump. This actuation does not, however, begin until after the engine speed has been increased from an idling speed. Further, through the same control concept, increases in engine speeds naturally bring about faster and faster operation of the motor, subject to particular controls as is set forth in my application supra.

As those skilled in the art will fully appreciate, increases in the loading of the hydraulic motor in an industrial truck, as when moving up a grade, will naturally result in increased loading of the hydraulic pump and the engine of the truck. So far as I am aware, in order to prevent overloading of the engine, control means are utiliz-ed to vary either the volume of the hydraulic motor or the volume of the hydraulic pump. This causes an Valmost constant adjustment of control means and must naturally result frequently in the loss of efficiency because the engine power is not effectively utilized.

As a particular feature of my invention, I do bring about a decrease in the volume of my motor in order to permit increased speed of the truck driven by the motor, all at the same engine speed, to permit faster movement of the truck under normal conditions, as is true of any vehicle. This is brought about by utilizing a variable volume motor vwith the motor volume normally held at maximum displacement to permit maximum torque at starting. Through the additional feature of utilizing the pressure at the low pressure 'side of `the motor, I so control the motor with this low pressure as to decrease its volume to permit speeding of the motor as the motor naturally tends to move faster. Most importantly, I do not allow adjustment of the motor towards increased displacement until after the pressures within the control system are increased to a predetermined ligure. In other words, should the truck move upwardly on a grade, and should the pressures increase, the motor will still be held with its displacement less than maximum so that the engine will actually be required to assume an increased burden. This increased burden will be assumed by the engine until after pressures are increased to a predetermined point. Only then, will the displacement of the motor be changed.

Those skilled in the art will thus appreciate that the engine of my truck will assume greater and greater burdens, just as does the engine in a conventional gear drive, until that burden reaches a predetermined point, at which .time there will be a shift in the control to lighten 3,304,710 Patented Feb. 2l, 1967 ICC the burden of the engine. Further, the burden on the engine will be proportional to the speed of the motor as well as to the pressure developed by the engine in driving the pump that, in turn, is actuating the motor. It is this relationship that is the basic concept of my invention.

As I have already set forth in describing the basic concept of my invention, a particular feature of my invention resides in the utilization of the pressure at the low pressure side of the motor for controlling the drive between the hydraulic pump and the motor, after the motor has naturally attained a particular speed.

I have thus outlined rather broadly the more important features of my invention in order that the detailed description thereof that follows may be better understood, and in order that my contribution to the art may be fully appreciated. There are, of course, additional features of my invention that will be described hereinafter and which will form the subject of the claims -appended hereto. Those skilled in the art will appreciate thatA the conception on which my disclosure is based may readily be utilized as a basis for the designing of other structures for carrying out the several purposes of my invention. It is important, therefore, that the claims be regarded as including such equivalent constructions as do not depart from the spirit and scope of my invention, in order to prevent the appropriation of my invention.

The drawing schematically illustrates applicants novel control system for a hydrostatic transmission as used on an industrial truck. The pump of the hydrostatic unit is indicated by the designation FP which indicates a xed displacement type of hydraulic pump. The motor of the hydrostatic unit is indicated by the designation VM which indicates a variable displacement type of hydraulic motor that is operably connected by linkage 51a with cylinder 51 to vary its displacement. Y

Illustrated in the drawing is the schematic arrangement of the hydraulic pump and motor constituting the hydraulic transmission. Although not shown, it is to be understood that the output shaft of the hydraulic motor VM is connected by suitable means to the driving wheels of the truck. v v

As explained briefly above, the hydraulic pump FP, is a xed displacement pump and is operably connected to the engine of the truck. Here again, any suitable driving means may be employed depending upon the intended use of the truck. For example, gasoline, diesel or electric motor means may be used. The type of prime mover used has no effect on the present invention. For our purposes it is sufficient that the prime mover be indicated by the designation E as shown in the drawing.

The pump FP, is driven by the engine E, and the speed of the engine E in its `relation to the torque load controls automatically the drive of hydraulic motor VM in a manner to be set forth hereinafter.

Assuming the engine E is idling and the system is suthciently charged with hydraulic fluid, then the directional spool DS, control valve CV, piston P and valve 60 with spools 53 and 61 will each be in the position illustrated in the drawings. Shuttle valve 15 will be to the left of the position shown in the drawing. The idling engine E develops pressure at the pump, this pressure is discharged into `chamber B. Fluid will exit from chamber B via passage 21. Inasmuch as check valve CVI is set at 4000 p.s.i., the hydraulic uid will not ow through the passage controlled by CVll` because the hydraulic pressure is not high enough to unseat the valve. Therefore, fluid flows via passage 21 into groove 12. With control valve CV in the position shown, fluid will pass within the bore containing control valve CV into the groove 22 and thence into passage 23. At this point,

shuttle valve 15 has already rmoved to the right, exposing passage 25 to the pressure within chamber A. Fluid will then flow out of chamber A through throat 11 toward the filter and back to the suction side of the pump. Check valve CV2 did not unseat as yet because this valve is set to unseat at 30 p.s.i. The purpose of throat 11 is to build up pressure in the system for effecting control of the hydraulic motor VM. Fluid in groove 22 will also flow to the right within the bore containing control valve CV, past shoulder 20 on control valve CV and through passage 36 into groove 35 into the radial bore 35a of directional spool DS, thence in the axial bore 35b and into the drain chamber. Fluid is returned to the suction side of the pump FP via passage 37.

At the `same time that fluid is being directed into passages 23 and 36 from groove 22, lluid is directed into passage 24 which supplies fluid to chamber C. However, there will be no movement of control valve CV until there has been a sufficient buildup of pressure in the hydraulic system. This recirculation of hydraulic fluid will continue until the pressure buildup is suflicient to move control valve CV to the right. As long as directional spool DS remains in its neutral position, as shown, the pressure in the system will never build up sufficiently to move control valve CV to the right because the fluid is constantly dumped into the drain chamber via passage 36, bore 35a and bore 35b. Control valve CV is maintained in spring-pressed position against washer 4, which is in turn held in position by spring 5. Control valve CV will move to the right upon the buildup of sufficient pressure in the system as indicated above. Also, control valve CV will move to the left with washer 4, from the position shown or any position rightward of this position, when the brake rod 6 is actuated through the foot treadle, schematically illustrated, by the operator of the truck.

Directional spool DS is illustrated in its neutral position in the drawing. It can be moved to the right or left (through linkage not shown), by the operator depending on the desired direction of travel. When directional spool DS is moved a sufficient distance to the right or to the left, it closes groove 2 at one side to direct .the flow of fluid to one side or the other of the motor VM via passage 30 or 31.- Hydraulic motor VM is provided with a passage 3 which is connected to the casing of lthe motor VM. This line 3 drains the casing of motor VM and returns any fluid that leaks from the motor VM back to the reservoir. l

With the engine E idling, the lluid which is being pumped continues to recirculate back to the suction side of the pump FP in the manner set forth above. It is to be noted that at this time no pressure fluid is being supplied to valve 60 because there is no communication at groove 50. However, lthe right side of piston P is subjected to drain chamber pressure, which is negligible, and the piston P is moved to maximum displacement by means of spring 49. The right side of piston P is interconnected with the drain chamber via passage 56, grooves 55D and 55 in outer spool 61, passage 54 of inner spool 53, charnber D, passage 52, groove 50, radial bore 16, axial bore 17 which interconnects with large bore 18 ofthe control valve CV and thence the drain chamber.

As pointed out above, this occursA prior to the full buildup of pressure in the hydraulic system with the directional spool DS still in its neutral position as illustrated.

Let us now assume that the operator wishes to move the vehicle in a forward or rearward direction. To accomplish this, he shifts the directional spool DS into a directional position to the right or left of the position illustrated in the drawing. He then speeds up the engine E so that he begins the development of rather considerable system pressure. As the engine E is accelerated, the pressure in chamber B increases. Shuttle valve 15 is to the right, in `the position shown in the drawing. Thus, fluid returning via passage 23 is directed into chamber A and fluid is discharged therefrom via throat 11 back to the suction side of the pump. The throat 11 acting as a restriction in .the fluid line helps to establish the operating pressure. The ball check valve CV2 which is spring biased, is set to release at 30 p.s.i. Once a pressure of 30 p.s.i. is established in the system, the valve CVZ will unseat and permit a bypassing of the throat 11. It may, therefore, be said that valve CVZ'limits the amount of control pressure, and cuts losses in power `as well as reducing the amount of heat generated by the hydraulic system.

The fluid now moving to the chamber C will force the control valve CV to the right against the pressure of its biasing spring 19 and moves it away from washer 4. This movement of control valve CV brings about a closure of groove 12 so that it is no longer in communication with groove 22. Fluid will now flow, therefore, from the groove 12 past the ball check valve CV3 into groove 2 and in one direction or another through the hydraulic motor VM, depending upon the position of directional spool DS.

For purposes of illustration, let us assume that directional spool DS is moved to .the right and the fluid flows out of groove 29 through passage 30 into the right side of the motor VM and then out of the motor through passage 31 and into the groove 32. From 4the groove 32 .the fluid will flow to the groove 33, passage 34, groove 35, passage 36, past the shoulder 20 of the control valve CV and back to the groove 22 and into control chamber C. In other words, when groove 12 is disconnected from groove 22, fluid to chamber C will come from the exhaust fluid returning from the motor VM.

It should be pointed out that when the truck is first started out, the motor VM is at maximum displacement thus providing maximum torque at the Wheels for starting. As the truck picks up speed, less fluid is required by the motor VM to maintain the speed, Athus the system pressure will increase sufficiently to move control valve CV to the right to expose groove 50. As this occurs, servo pressure is discharged through groove 50 and is conducted via passage 52 into chamber D. With valve 60 in .the position shown, fluid enters central bore of inner spool 53 and exits via radial bore 54, through groove 55, passage 56 into the cylinder 51 and acts against the right side of piston P to move the piston P leftward to decrease the displacement of the motor VM and permit it to speed up. It can be seen that the motor VM will automatically be adjusted to compensate for slight grades up which the truck is traveling.

Let us assume now, that the truck is traveling up a steep grade, and the operator accelerates the engine to maintain the speed of travel. By accelerating the engine, the pressure in chamber B is increased sufficiently to move piston 40 to the left. In doing so, inner spool 53 was also moved to the left. The movement of inner spool 53 is sufficient to bring groove A into communication with radial opening 54A in the spool 53, and simultaneously, completing the connection of passage 57 with inner spool 53. Therefore, control fluid will flow from chamber C into groove 50 and down through passage 52 into the bore of inner spool 53 of valve 60. The fluid will flow through the central bore of spool 53 and radially outwardly through 54A, thence into groove 55A and passage 57 so as to move the piston P within the cylinder to bring about an increase of the displacement of motor VM through linkage 51A. This will, naturally, cause the motor VM to operate at lower speed in proportion to the pump FP volume thereby permitting the vehicle to climb the upgrade. Fluid will flow from the opposite side of piston P in cylinder 51 through the passage 56 through grooves 55D and 55C and down through passage 58 to the reservoir.

It lwill be well to note that spring 63 which biases inner valve spool 53 is of such strength as to resist -any movement of the spool 53 away from its normal position when the engine is idling until the development of a particular operating pressure at the pump FP. In other words, until a particular operating pressure is encountered at the pump, the pressure will continue to increase as the engine E develops speed and as pressure is created at the throat 11. However, upon the system reachinga critical pressure, which is predetermined, small piston 40 will be moved to the left in the manner outlined above.

Before describing further operational feat-ures of my novel system, I should like to indicate that check valve CV3 is utilized to hold the motor -against rotation in a direction reverse to drive. In other Words, if the motor VM is in a particular directional drive position, should the operator stop the vehicle as it is facing up a grade, the deceleration of the engine and pump to the minimum speed will not permit the vehicle to slip downhill. This is due to the fact that the check valve CVS does not allow reverse ilow of fluid from groove 2. The same is true, re- Vgardless of the direction the truck is moving.

Reference character CV6 designates a relief valve which lwill unseat at` 2000 p.s.i. In the event that the operator should accidentally change the drive direction, the immediate increase in pressure will unseat the valve CV6 and there will be a bypassing of fluid from valve CV3, passage 13A, passage 141, valve CV6 and back to groove 12, toward pumpiFP, thence CVI permits escape of additional oil back to the suction side of pump FP.

It will be appreciated that when the vehicle is moving downhill, the motor will tend to drive faster than the speed due to the uid it receives from the pump FP. In that event, the additional fluid flow generated by the speedup of the motor VM will merely recirculate through the valve CV7, bypassing the groove 22 from passage 34, groove 35, passage 36 to valve CV7.

It will be appreciated further that when the motor acts as a pump in moving downhill, it` may eventually build up very considerable pressure. If so, the fluid under pressure will not completely recirculate through valve CV7 but will move also into chamber A through passage 70 after moving past shoulder 20. The development of this pressure will move shuttle valve 15 to the left. At this point, the throat 11 and valve CVZ vare no longer in communication with passage 23. Therefore, any uid generated by the pump FP `must now exceed 50 p.s.i. since the return fluid to the suction side of the pump FP must necessarily ilow past check valve CV4 `which is set for 50 p.s.i. This means that the pump FP must develop 50 p.s.i., this pressure builds up very rapidly. The development of this additional pressure will act immediately on the motor intake to prevent cavitation. Naturally, this quickly developed additional pressure also augments circulation so that cavitation is very effectively resisted.

For purposes of review, again assume that the vehicle is moving uphill, and that due to an acceleration of the engine E, a high pressure has been developed at the pump, this pressure being higher than the critical pressure for which spring 63 has been calculated. The small piston 40 will now move the inner spool 53 to the left of its position shown. At the same time, since control valve VC is already in a position exposingithe lgroove 50, the fluid pressure developed at chamber C will flow from the control chamber C through groove 5d and passage 52 into the central bore of inner spool 53, out through radial passage 54A, groove 55A, groove 55B and through passage 57 so as to move the control piston toward maxim-um displacement of the motor. Now there will be greater torque available for the wheels to compensate for upgrade climb of the vehicle permitting it to continue upgrade at substantially the same hydraulic pressure. This means that the load transmitted through the pump to the engine wil-l be substantially unchanged and no loss of horsepower output will follow. Therefore, the engine should maintain a constant r.p.m. In other words, the inner spool 53 is capable of moving back and forth into and out of a position related to the maximum and minimum displacement of the motor VM. Because of the fact that the spring 63 is set at a critical control position, all related to the power of the engine and the displacement of the motor, there will be no movement of the inner valve spool 53 under normal` operating conditions. The motor being under those normal operating conditions in a position predisposed toward maximum displacement-as is determined by the control pressure generated. The reason for having maximum displacement when starting ott is to provide the motor VM with maximum torque.

Should the grade up which the vehicle is traveling be so severe as to require the development of pressure by the pump greater than 4000 p.s.i., check valve VCI, which is set for 4000 p.s.i., will be moved off its se-at and there 'will be a bypassing of the pressure fluid back to the suction Side of the pump with the vehicle coming toa stop.

There is la hoist pump HP shown in thedrawing which is also driven by the engine E, when the hoist is not utilized for hoisting, this pump directs a portion of the ow of fluid to a lower lter and toward the engine driven traction pump FP whereby to replenish the operating system.

The brake rod is designated'by reference numeral 6 and when the foot treadle is depressed by the operator, the rod 6 moves control valve CV against the force of the spring 5 to unseat the washer 4. During this movement it closes the opening at shoulder 20 so that fluid cannot return from the motor VM except through the check valve CVS which is set at 2600 p.s.i. A metering notch to prevent an abrupt shutting olf of the flow of fluid is provided at 20A. Naturally, in order to overcome the 2600 p.s.i. setting, the pressure must first build -up in the hydraulic lines back t-o the motor VM, the effect of this lwill be to Islow ldown the motor and in fact, considerable braking is obtained.

At the same time, the groove 12 is opened so that there is -a bypassing of the motor VM by the pressure Huid coming from the pump FP through passage 21. The relationship between the opening of the groove 12 and theV closing of the fluid flow at 20 is avery unique one. It can readily he seen that it is possible to obtain inching through utilization of this construction in a particular manner. Inching in the industrial truck art means the operation of the engine E and pump FP in such :a manner that the truck moves extremely small distances while the engine operates at relatively high speeds.

Inching is particularly important where the load is being lifted into position las the truck is maneuvered. In this construction, there is an inching treadle ywhich opcrates the brake rod 6 in a very sensitive manner to effect a very delicate control at 12 and 2G. This is all accomplished without actually operating the wheel brakes. A second treadle is utilized for operating the lbrake rod 6 to obt-ain the control effect `at 12 and 2t), while also operating the Wheel brakes.

From the preceding description, it can be seen that there is provided a relatively simple, inexpensive drive which :allows the operator of the truck to smoothly and effectively control the operation of the truck through the accelerator of the engine. While certain forms of the invention have been shown and described, it will be appreciated that this is for the purpose of illustration and that changes and modi'cations may be made ,theerin without departing from the spirit and scope of the invention set forth.

I now claim:

1. In a combination of the class described, an engine driven pump, a control valve in communication with said pump for receiving fluid from said pump, a bypass passage through which uid may flow from the pressure side of said pump to the low pressure side thereof, a flow retarder in said bypass passage adapted to develop pressure at the pump pressure side of said flow retarder so as to present fluid pressure to said control valve to move said valve, a pair of relief valves of different pressure settings for said ow retarder, and means for controlling which of said relief valves limits the degree of fluid pressure developed by said fiow retarder.

2. In a combination of the class described, an engine driven pump, a variable displacement hydraulic motor driven by fluid discharged under pressure by said pump, a control valve in communication with said pump and controlling theow of fluid from said pump to said hydraulic motor, a bypass through which fiuid may fiow from the discharge side of said pump to the suction side thereof, a fiow retarder in said ,bypass adapted to develop pressure at the pump discharge side of said ow retarder so as to present fiuid pressure to said control valve to move said valve, a pair of relief valves of different pressure settings for said liow retarder, a shuttle valve operably mounted within a bore connected to said bypass, one end of said shuttle valve subjected to the discharge pressure of said pump, the other end of said shuttle valve subjected to the discharge fluid from said hydraulic motor whereby in the event that said hydraulic motor is overdriven said shuttle valve moves to operably connect said relief valve of higher setting with said bypass to increase the system pressure thus insuring fluid at said motor inlet at all times thereby preventing cavitation.

3. In a combination of the class described, an engine driven pump, a variable displacement hydraulic motor driven by fiuid moving to it from said pump, a hydraulic positioning means for controlling the displacement of said hydraulic motor, hydraulically actuated control means for directing fiuid to said motor from said pump adapted for actuation automatically when said pump develops fiuid pressure directly proportional to the acceleration of said engine, directing means for directing a portion of said iiuid under pressure to said hydraulic positioning means for controlling said motor displacement to hold it at minimum displacement until said pressure increases to a critical pressure, said critical pressure effecting the movement of said positioning means toward a position corresponding to maximum displacement of said motor.

4. In a combination of the class described, an engine driven pump, a variable displacement hydraulic motor driven by fluid moving to it under pressure from said pump, a hydraulic positioning means for controlling the displacement of said hydraulic motor, a first control valve for controlling the fiuid supplied to said hydraulic positioning means, said first control valve moving into open position when the fluid pressure in the system reaches a predetermined operating level, a second control valve for controlling the flow of uid permitted by said first control valve, said second control valve directing fiuid to said hydraulic positioning means to move and hold said hydraulic positioning means toward minimum displacement when said uid pressure reaches said operating level, said first and second control valves remaining in said positions until a predetermined critical system pressure is attained, and means responsive to said critical pressure for effecting the movement of said positioning means toward a position corresponding to maximum displacement of said motor.

5.v In a combination of the class described, an engine driven pump, a variable displacement hydraulic motor driven by iiuid discharged under pressure from said pump, a directional spool valve for controlling the direction of said hydraulic motor, a hydraulic positioning means for controlling the displacement of said hydraulic motor, biasing means normally urging said positioning means toward maximum displacement, a first control valve for controlling the fiuid supplied to said positioning means, bypass means controlled by said first control valve for directing the pressure fiuid back to the suction side of said pump until a predetermined operating pressure is attained, restriction means in said bypass means to control the rate at which said operating pressure is attained, said first control valve moving into its open position when the fluid pressure in the system reaches said predetermined operating level and simultaneously cutting off said bypass means, a second control valve in series with said first control valve for controlling the flow of fluid permitted by said first control valve, said second control Valve directing fluid to `said hydraulic positioning means to move and hold said hydraulic positioning means toward minimum displacement when said fiuid pressure reaches said operating level, said first andsecond control valves remaining-in said positions until a predetermined critical pressure is attained, and means responsive to said critical pressure for effecting the movement of said second control valve whereby said positioning means moves toward a position corresponding to maximum displacement of said motor.

6. In a combination of the class described, an engine driven pump, a variable displacement hydraulic motor driven lby pressure fluid moving to it from said pump, a hydraulic positioning means for controlling the displacement of said hydraulic motor, a first control valve for controlling the fiuid supplied to said hydraulic position ing means, said first control valve moving into open position when the fluid pressure in the system reaches a predetermined operating level, means for subjecting said first'control valve to the discharge fiuid of said motor after said first control valve has moved into said open position, a second control valve for controlling the fiow of fluid permitted by said first control valve, said second control valve directing fluid to said hydraulic positioning means to move and hold said hydraulic positioning means toward minimum displacement when said tiuid pressure reaches said operating level, said first and second control valves remaining in said positions until a predetermined critical system pressure is attained, and means responsive to said critical pressure for effecting the movement of said second control Valve whereby said positioning means moves toward a position corresponding to maximum displacement of said motor.

7. In the combination of a vehicle having an engine, a hydraulic pump driven by `said engine, a variable displacement hydraulic motor for driving said vehicle and operable when fiuid is supplied to it under pressure from said pump, a hydraulic positioning means for controlling the displacement of said hydraulic motor, a control valve for controlling the fiuid supplied to said hydraulic positioning means, said control valve moving into open position when the fluid pressure in the system reaches a predetermined operating level, passage means downstream of said control valve for supplying fluid to said hydraulic motor when said first control valve moves into said open position, means located in said passage means for preventing reverse rotation of said hydraulic rnotor in the event of a loss of system pressure, said last mentioned means preventing movement of said vehicle in a direction opposite to that of which it was traveling immediately prior to the loss of system pressure.

8. In the combination of a vehicle having an engine, a hydraulic pump driven by said engine, a variable displacement hydraulic motor for driving said vehicle and operable when fluid is supplied to it under pressure from said pump, a hydraulic positioning means for controlling the displacement of said hydraulic motor, a control valve for controlling the fluid supplied to said hydraulic positioning means, said control valve moving into open .position when the fiuid pressure in the system reaches a predetermined operating level, passage means downstream of said control valve for supplying fiuid to said hydraulic motor when said first control valve moves into open position, a direction spool for controlling the direction of ow of .pressure fluid to said hydraulic motor, first means located in said passage means for preventing reverse rotation of said hydraulic motor in the event of a loss of system prsure, second means in saidv passage means for relieving excessive pressure in said passage means when th@ QOSAitionY of said direction spool is suddenly changed-,-

said second means permitting prsure fluid to bypass said first means and return to the suction side of said pump.

9. In the combination of a vehicle having an engine, a hydraulic pump driven by said engine, a variable displacement hydraulic motor for driving said vehicle and operable when fluid is supplied to it under pressure from said pump, iirst means for establishing a first operating pressure level in said system, valve means responsive to said first operating pressure level permitting uid to pass to said motor, return passage means from said motor to the suction side of said pump, second means for establishing a second operating pressure level in said system, said second operating pressure level higher than said irst operating pressure level, whereby when said vehicle is traveling downgrade and said motor overdrives thus building up pressure, said pressure buildup in the discharge fluid from said motor activates said second means for establishing a higher operating pressure in said system thus preventing cavitation of said motor by ensuring sufficient tiuid at the inlet thereof.

10. In the combination of a vehicle having an engine, a hydraulic pump driven by said engine, a variable displacement hydraulic motor for driving said vehicle and operable when fluid is supplied to it under pressure from said pump, hydraulic positioning means for controlling the displacement of said hydraulic motor, a first control valve having rst means for controlling the fluid supplied to said hydraulic positioning means, said first control valve also having second means for controlling the supply of pressure fluid to said hydraulic motor, said rst control valve having third means for controlling the tiow of discharge uid from said motor back to the suction side of said pump, hydraulic motor braking means associated with said first control valve, actuating means for moving said rst control valve in a direction to move said third means to closed position cutting off the normal return ow of motor discharge fluid and direct the iiuid through passage means provided with a preset relief valve, the set-ting of which is considerably higher than the pressure of returning Huid, thus halting the flow of return fluid until the pressure overcomes the setting of said relief valve thereby eliecting considera-ble vbraking of said vehicle through the action of said hydraulic motor.

11. In the combination described in claim wherein said irst means for controlling the fluid supplied to said hydraulic positioning means comprises a land at one end thereof which controls the opening to the passage leading to said hydraulic positioning means.

12. In the combination described in claim 10 wherein l@ said second means for controlling the supply of fluid to said hydraulic motor comprises a shoulder portion located intermediate said rst and third means on the body of said valve which controls the recirculation of pressure tiuid back to said pump.

13. In the combination described in claim 1t) wherein said third means for controlling the flow of discharge fluid from said motor back to said pump comprises an annular shoulder at a location removed from said irst means, said shoulder cutting ott the return of uid from said hydraulic motor and directing said return fluid through said passage containing said preset relief valve.

14. In a combination of the type described in claim 13 wherein said annular shoulder is further provided with a metering notch which provides for a gradual cutting ott of the return fluid from said hydraulic motor.

15. In a combination of the type described in claim 10 wherein said actuating means for moving said rst control valve in a direction to move said third means to closed position comprises an actuating rod which is centrally mounted within said first control valve and connected through linkage to a foot treadle on said vehicle.

16. In the combination of an industrial truck having an engine, a hydraulic pump driven by said engine, a variable displacement hydraulic motor for driving said vehicle, a hydraulic pressure system for conveying said iiuid from said pump to said hydraulic motor and back to the suction side of said pump, a hoist pressure circuit, a control valve in communication with said hydraulic pressure system for controlling the fluid supplied to said hydraulic motor, said control valve also controlling the return of fluid from said hydraulic motor to the suction side of said pump, means for maintaining the hydraulic fluid in said hydraulic pressure system at a proper operating level to automatically replenish any hydraulic uid losses that the system may encounter, said last mentioned means comprising an auxiliary pump driven by said engine, passage means connecting the discharge of said means for directing said auxiliary pump discharge into said suction side of said first mentioned pump when said hoist pressure circuit is not in use.

References Cited by the Examiner UNITED STATES PATENTS 2,942,421 6/1960 Hann et al. 60-19 2,941,365 6/196()y Carlson et al. 60-19 3,153,900 10/1964 Pigeroulet et al. 60-19 EDGAR W. GEOGHEGAN, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTlON Patent No 3, 304 7l() February 2l, 1967 George F. Quayle ror appears in the above numbered pat- It is hereby certified that er the said Letters Patent should read as ent requiring correction and that corrected below.

line 39, before means" insert auxiliary Column l0,

n side of said first mentioned pump and pump with the suctio valve Signed and sealed this 7th day of November 1967 (SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Ir.

Commissioner of Patents Attesting Officer 

1. IN A COMBINATION OF THE CLASS DESCRIBED, AN ENGINE DRIVEN PUMP, A CONTROL VALVE IN COMMUNICATION WITH SAID PUMP FOR RECEIVING FLUID FROM SAID PUMP, A BYPASS PASSAGE THROUGH WHICH FLUID MAY FLOW FROM THE PRESSURE SIDE OF SAID PUMP TO THE LOW PRESSURE SIDE THEREOF, A FLOW RETARDER IN SAID BYPASS PASSAGE ADAPTED TO DEVELOP PRESSURE AT THE PUMP PRESSURE SIDE OF SAID FLOW RETARDER SO AS TO PRESENT FLUID PRESSURE TO SAID CONTROL VALVE TO MOVE SAID VALVE, A PAIR OF RELIEF VALVES OF DIFFERENT PRESSURE SETTINGS FOR SAID FLOW RETARDER, AND MEANS FOR CONTROLLING WHICH OF SAID RELIEF VALVES LIMITS THE DEGREE OF FLUID PRESSURE DEVELOPED BY SAID FLOW RETARDER. 